The objective of this program is to isolate, identify and characterize human angiotensin I converting enzyme from lung and other body tissues, including vascular smooth muscle, red cell, plasma and kidney. We have found that human lung and red cells as well as plasma contain angiotensin I converting enzyme. Human lung converting enzyme is not inhibited by EDTA and does not require chloride for activation. Lung converting enzyme activity has been found at molecular weights of 600,000, 450,000 and 300,000 fractions of the effluent from a 6-B sepharose column. All three molecular weight fractions behave similarly in that they are not inhibited by EDTA. They are inhibited by phenylmethylsulfonylfluroide and para-hydroxymercuribenzoic acid. Cysteine and 2,2'dipyridyl inhibited the 600,000 and 450,000 molecular weights at 10 to the minus 2nd power M but not at 10 to the minus 4th power M. 2-mercaptoethanol, dithiothreitol and dithioerythritol does not inhibit any of the fractions. Iodoacetate does inhibit activity at 450,000 preparation. The prostaglandins E, A and F are not inhibitory but an angiotensin II analogue, 1,sarcosine,8-isoleucine angiotensin II, does inhibit the coverting enzyme activity as does standard human 5-isoleucine angiotensin II. Another angiotensin II analogue, 1,sarcosine,8-alanine angiotensin II, is not inhibitory. Red cell extract was found to contain converting enzyme activity at molecular weight 300,000 and preliminary enzymatic inhibitory studies indicate that it behaves much the same way as the lung fractions. Further purification studies are presently underway.